Will the JB weld work for this?
If you have a condenser inside the vehicle, you've got the most oddball air conditioner I've ever heard of. The condenser is that part out in front of the radiator where the hot vapor pumped by the compressor gets turned back into a liquid. The evaporator(s) are what's inside to do the cooling. A condenser operates at around 250 PSI or so, while an evaporator's normal pressure is about 25-30 PSI. No problem so far- - - -until the engine stops and the pressure in the whole system equalizes at 100+ PSI. I don't know of any sort of epoxy that will contain that kind of pressure.
Problem #2- - - - -a pinhole leak in a refrigeration line is like an iceberg- - - - -by the time it starts leaking, the damage inside is cone-shaped and you're only seeing the tip of the ice cream cone. It will continue to deteriorate even if you manage to get the leak plugged temporarily. Bite the bullet and install a new EVAPORATOR. Chances are the condenser is still OK.
Evan- - - -if you use bottled nitrogen like the commercial refrigeration shops do, or even CO2 for a MIG welder to pressure check a new system, you don't have to worry about introducing moisture into the system with compressed air. The 2200 PSI in either bottle can be reduced to any test pressure you want with a welding tank regulator.